The present invention relates to a black pigment powder that is-safe and that has no harmful effect on either the environment or the human body; and that is also free of magnetic aggregation. More specifically, the present invention provides a black pigment powder with good dispersibility and high blackness for use as a colorant of a toner, an electrophotography developer, or as a colorant of resins including a coating and a cosmetic material.
Methods of development in electrophotography can be generally divided into a unitary development method, in which development employing a toner itself with a colorant dispersed in a binder resin is employed, and a binary development method, in which development is performed by mixing a toner with a carrier, to thereby cause the toner being carried by the carrier.
In either case, in reproduction, an electrostatic latent image formed on a photosensitive material is developed using one of these toners, and a toner powder having performed image formation on the photosensitive material is transferred onto a transfer material such as a paper or a sheet, and is fixed utilizing heat, pressure or the like to obtain a permanent image. In these toners, a black colorant such as carbon black and magnetite is employed in large amounts to provide an image-forming material. Carbon black is employed in both the binary development method and a non-magnetic unitary development method, while magnetite is employed in the magnetic unitary development method whereby a toner is carried by magnetism.
With the development of computers there has been a corresponding increase in the demand for toners as image-forming materials, and in the use of colorants used in such toners.
However, damage to the environment and to health is a matter of great concern, and consequently, there has been an increasing demand for the safety of raw materials used in toners.
Carbon black, which is employed in large amounts as a black colorant of a toner, contains a small amount of an aromatic hydrocarbon, which contains 3,4-benzpyrene, a substance which has been identified as being carcinogenic. There is therefore an increasing concern in the society about the safety of toners in which carbon black is employed.
In contrast, magnetite is a black pigment comprising safe and harmless iron oxide as a main component, but it suffers from a problem in that since the magnetite particle tends to re-aggregate due to its high value of magnetization, thereby making it difficult to obtain a uniform dispersion. Besides, due to its electrically-conductive properties, it cannot be employed as a colorant of a toner for use in either the binary development method or in the non-magnetic unitary development method requiring insulation properties or high resistance.
As non-magnetic black pigments as a substitute for carbon black, a black pigment particle powder consisting of polycrystalline particles having a mixed composition of Fe2TiO5 and a Fe2O3—FeTiO3 solid solution is described in the official gazette of Japanese Patent Public Disclosure (Kokai) No. 2276/1991; and black particle powders having an Mn-containing hematite structure are described in the official gazettes of Japanese Patent Public Disclosure (Kokai) No. 143316/1996 and Japanese Patent Public Disclosure (Kokai) No. 10344/2000.
The black pigment described in Japanese Patent Public Disclosure (Kokai) No. 2276/1991 is a black pigment particle powder consisting of polycrystalline particles having a mixed composition of Fe2TiO5 (pseudobrookite) and a Fe2O3 (hematite)-FeTiO3 (ilmenite) solid solution obtained by coating a magnetite particle powder with a titanium compound or calcining a mixed powder of a magnetite particle powder and a titanium compound in a non-oxidative atmosphere; since it comprises harmless iron oxide and titanium oxide as main components, its use does not give rise to problems of safety. However, it suffers from a drawback in that it lacks blackness. Moreover, since the black pigments described in Japanese Patent Public Disclosure (Kokai) No. 143316/1996 and Japanese Patent Public Disclosure (Kokai) No. 10344/2000 contain a large amount of Mn, an environmental pollutant, they cannot be regarded as harmless pigments. In the official gazette of Japanese Patent Public Disclosure (Kokai) No. 253717/2001 there is described a process for obtaining an iron-titanium composite oxide particle powder by a spray thermal decomposition method employing an emulsion containing an aqueous ferrous salt solution, a hydrolytic organic titanium compound and an emulsifier as a spray thermal decomposition solution. However, the process cannot be utilized with existing pigment-producing equipment and requires the use of a spray thermal decomposition device, and is therefore costly.
Thus, to date a black pigment that harms neither the environment nor health, and which has good dispersibility as a substitute for carbon black which has been employed in large amounts as a colorant of a toner for the binary development method and the non-magnetic unitary development method, has not been-produced. There is a great demand for the development of such a black pigment; and such a pigment should also be able to be employed not only as a colorant for a toner but also in resins, including coating materials and cosmetic materials.